The present invention relates, generally, to a three-dimensional structure forming method, and more particularly, to a method for forming a three-dimensional structure on a thick, photosensitive resin.
Recent demand for smaller and lower-profile electronic devices has increasingly required finer semiconductor devices to be mounted onto these electronic devices, and a critical dimension has become smaller than 0.15 μm. For this requirement, various proposals have been made to improve resolving power of a projection exposure apparatus.
The resolution improves effectively with increased numerical aperture (“NA”) of the projection optical system or higher NA and a shortened wavelength of an exposure light source. Therefore, exposure light sources have recently been in transition from the KrF excimer laser (with a wavelength of approximately 248 nm) to the ArF excimer laser (with a wavelength of approximately 193 nm), and the F2 excimer laser (with a wavelength of approximately 157 nm), which have been almost reduced to practice.
The projection optical system is subject to chromatic aberration that deteriorates imaging performance because a glass material has different indexes of refraction according to light wavelengths. Therefore, a projection exposure apparatus that uses the KrF excimer laser as an exposure light source narrows its band to emit a single beam. A projection exposure apparatus that uses the ArF excimer laser as an exposure light source employs, for achromatism, two types of glass materials, such as quartz (SiO2) and calcium fluoride (CaF2), for an optical system.
However, the F2 laser for use with an exposure light source limits the light transmitting glass material. Currently, CaF2, magnesium fluoride (MgF2), lithium fluoride (LiF), etc. are those glass materials, which provide the desired transmittance. However, only CaF2 is a viable glass material, which provides an optical system in the projection optical system with the necessary uniformity and a large aperture of the crystal. Therefore, the achromatism is not available through two kinds of glass materials, unlike the projection exposure apparatus that uses the ArF excimer laser as an exposure light source.
Accordingly, there has been proposed a projection optical system that uses for achromatism a catadioptric system including a mirror as well as lenses (see, for example, Japanese Laid-Open Patent Application No. 2001-228401). Such a projection optical system requires the mirror not to shield the light, and thus typically utilizes an arc imaging area having a certain height from the optical axis.
A projection optical system that projects a pattern on a mask (or a reticle) onto a substrate that applies a photosensitive agent, through a projection optical system that forms an arc imaging area, needs an illumination apparatus for illuminating a mask with an arc illumination area. Typically, a rectangular slit 1000 shown in FIG. 14 is illuminated, and an arc opening 1100 and a light shielding part 1200 take out an arc illumination area. Here, FIG. 14 is a schematic plane view showing one example of the slit 1000 for taking the arc illumination area out of the rectangular illumination area.
However, in taking the arc illumination area out of the rectangular illumination area, the slit shields light and lowers illumination efficiency, and cannot obtain high light intensity on the photosensitive substrate. As the light intensity on the photosensitive substrate decreases, the exposure time increases and the circuit-pattern transfer per unit time, or throughput, decreases. Therefore, the high light intensity is necessary on the photosensitive substrate.
For enhanced light intensity on the photosensitive substrate, there have been provided a method that uses an optical fiber (see, for example, Japanese Laid-Open Patent Publication No. 5-68846), and a method that uses an arc fly-eye lens that has an arc outline of an element lens (see, for example, Japanese Laid-Open Patent Publication No. 62-115718). However, a method that uses an optical fiber has a practical difficulty because it cannot make the light sufficiently uniform through the optical fiber, and no optical fiber can handle a wavelength of 157 nm.
On the other hand, it is tremendously arduous to manufacture the arc fly-eye lens by processing rod lenses and cutting each outline into an arc shape. In addition, the manufacture tends to produce a large process error. As a consequence, the arc fly-eye lens becomes expensive, and the method that uses the arc fly-eye lens also has a practical difficulty, because it accumulates process errors when piling up respective element lenses, and deteriorates the performance of the entire lens. Accordingly, there has recently been proposed a method for processing a micro lens array using photolithography. In particular, a method for making the arc fly-eye lens with the micro lens array has attracted attention.
Photolithography can be used to relatively inexpensively form an arc fly-eye lens from a micro lens array, with a reduced processing error. This is because it results only from an alignment error of an exposure apparatus, and the error does not accumulate, because the element lens are not piled up. Therefore, this lens hardly deteriorates the entire performance.
According to the photolithographic technique, a two-dimensional circuit pattern formed by a combination of opening and light-shielding parts is transferred onto a photosensitive resin or photoresist etc., and it does not usually depend on a height direction of the circuit pattern or a three-dimensional shape. There has been provided a method that controls the shape in the height direction by partial exposure-amount adjustments and forms a three-dimensional shape on the photoresist (see, for example, Japanese Laid-Open Patent Application No. 63-289817). Moreover, there has been proposed a method that can be used to etch an optical element with a three-dimensional photoresist and manufacture a three-dimensional optical element (see, for example, Japanese Laid-Open Patent Application No. 2002-287370).
The micromechanics, such as a biochip, and an optical element, such as a micro lens array, have recently been required to have a special surface shape. Even in this case, a surface shape of a three-dimensional structure is formed by exposing a photoresist formed on a substrate, with a mask having a two-dimensional transmittance distribution, and then developing the photoresist.
When lithography is used to form a three-dimensional structure on the photoresist as a photosensitive resin, a thickness of the photosensitive resin determines the height of the three-dimensional structure. The photosensitive resin, thickness and selectivity with the substrate in etching define a transfer of a photoresist's shape onto the substrate through anisotropic etching, etc. The three-dimensional structure's height needs to range from sub-micron to about several hundred microns depending on purposes of use, such as a micro lens array.
Instant applicants have discovered that in forming a photosensitive resin with a three-dimensional structure through exposure with a distributed exposure energy amount, a structure shown in FIG. 15 appears on and remarkably roughens the surface, because almost all the parts of the photosensitive resin are developed during the formation of the three-dimensional structure. In other words, after the photosensitive resin is applied on the substrate, baking is performed to remove the solvent, and baking generates the Bernard convection. The Bernard convection produces Bernard cells in the photosensitive resin, and the Bernard cells appear on the surface due to development. Here, the Bernard convection is a thermal convection that induces a pattern called Bernard cells, as a static and horizontal fluid layer is uniformly heated from the bottom, and a difference in temperature between the upper and bottom surfaces reaches a certain extent. Here, FIG. 15 is a schematic plane view showing exemplary Bernard cells generated in the photosensitive resin.
In a method that is used to transfer a three-dimensional structure onto a substrate using etching, it is conceivable to increase the etching selectivity between the photosensitive resin and the substrate, and to form a desired three-dimensional structure even in a photosensitive resin having a small film thickness. However, this method would result in a rougher photosensitive resin shape and newly roughens the shape of the surface.